A relay device called a repeater or the like may be installed for improving communication quality in the radio communication and expanding the service area. The relay device receives a signal transmitted from a base station, and amplifies and relays the signal to a terminal, allowing the terminal to communicate even at an area where a radio wave from the base station hardly reaches. The relay device also amplifies and transmits a transmission signal of the terminal to the base station. The number of relay devices included in a communication path between the base station and the terminal is optional.
As a related art, a relay device is known that variably selects one polarization direction of at least one antenna according to the isolation between a donor antenna which transmits and receives signals to and from the base station and a service antenna which transmits and receives signals to and from a mobile station (for example, Japanese National Publication of International Patent Application No. 2009-521889). Also, a satellite automatic tracking antenna is known that rotatably drives, according to the reception signal level, a linear polarization flat antenna and a beam reflector plate which reflects an antenna beam of the flat antenna in the direction of satellite elevation angle (for example, Japanese Laid-Open Patent Publication No. 8-307136). Also proposed is a technique which suppresses sneaking of a signal from a transmission antenna to a reception antenna by adjusting the gain of a second antenna group in the direction of a first antenna group in a relay device comprising the first antenna group and the second antenna group (for example, Japanese Laid-Open Patent Publication No. 2006-020211). Also proposed is a control method of increasing the gain within a range where no oscillation occurs, based on the difference between an input wave and a sneaking wave by synthesizing a cancellation wave canceling the sneaking wave into the input wave and the sneaking wave in a relay device (for example, Japanese Laid-Open Patent Publication No. 2000-286772). Also a technique of adjusting the polarization angle of the demultiplexer based on the level of a vertical beacon signal and a horizontal beacon signal in a converter incorporating the demultiplexer is known (for example, Japanese Laid-Open Patent Publication No. 11-274962).
When communication between a base station and a terminal is relayed via multiple relay devices, a radio wave transmitted from a certain relay device may sneak to a reception antenna other than a relay device at the relay destination. A relay device with a reception antenna to which a transmission wave sneaks reduces the transmission gain to mitigate interference. As a result, an area covered by the relay device is narrowed. Sneaking of the radio wave among multiple relay devices is difficult to be mitigated by any of the related arts.